System and method for the reacquisition of a gated pilot

ABSTRACT

Systems and techniques are disclosed wherein a gated pilot signal can be re-acquired faster by searching a last known pilot offset and/or searching a last coset in which the last pilot signal was found.

BACKGROUND

[0001] 1. Field

[0002] The present disclosure relates generally to communicationssystems, and more specifically, to systems and techniques for thereacquisition of a gated pilot signal.

[0003] 2. Background

[0004] Modern communications systems are designed to allow multipleusers to share a common communications medium. One such communicationssystem is a code division multiple access (CDMA) system. The CDMAcommunications system is a modulation and multiple access scheme basedon spread-spectrum communications. In a CDMA communications system, alarge number of signals share the same frequency spectrum and, as aresult, provide an increase in user capacity. This is achieved bytransmitting each signal with a different pseudo-noise (PN) code thatmodulates a carrier, and thereby, spreads the spectrum of the signalwaveform. The transmitted signals are separated in the receiver by acorrelator that uses a corresponding PN code to despread the desiredsignal's spectrum. The undesired signals, whose PN codes do not match,are not despread in bandwidth and contribute only to noise.

[0005] In a CDMA communications system, a subscriber station may accessa network, or communicate with other subscriber stations, through one ormore base stations. Each base station is configured to serve allsubscriber stations in a specific geographic region generally referredto as a cell. In some high traffic applications, the cell may be dividedinto sectors with a base station serving each sector. Each base stationtransmits a continuous pilot signal which is used by the subscriberstations for synchronizing with a base station and to provide coherentdemodulation of the transmitted signal once the subscriber station issynchronized to the base station. The subscriber station generallyestablishes a communications channel with the base station having thestrongest pilot signal.

[0006] Since a continuous pilot signal requires bandwidth that couldotherwise be used to transmit information, some recently developed CDMAcommunications systems, such as IS-856, have employed gated pilotsignals. A gated pilot signal is characterized by a short period oftransmission of pilot signal followed by a long period of notransmission. By gating the pilot signal, additional bandwidth can berealized which increases the capacity of the base station. However,acquisition of gated pilot signal takes a longer time than acquisitionof a continuous pilot signal. If the mobile performs frequentre-acquisitions, long delays wasted for re-acquisition can deterioratethe quality of data service offered by the subscriber stations. Thus,what is needed is a system and method to reduce the time it takes tore-acquire a gated pilot signal after it has been lost.

[0007] It would be apparent to those skilled in the art that anycommunications system and method that uses a gated pilot signal canbenefit from a system and method to reduce the time it takes tore-acquire the gated pilot signal.

[0008] Systems and techniques for acquisition of a gated pilot aredescribed in U.S. Patent Application Number, “Acquisition of a GatedPilot,” Ser. No. 09/927,869, filed Aug. 9, 2001. Systems and techniquesfor acquisition of a gated pilot are disclosed wherein a gated pilotsignal can be acquired by searching for a first gated pilot signal,deriving timing information from the search for the first gated pilotsignal, and searching for a second gated pilot signal using the timinginformation.

[0009] Systems and techniques for acquisition of a gated pilot byavoiding partial correlation peaks are described in U.S. PatentApplication Number, “Acquisition of a Gated Pilot By Avoiding PartialCorrelation Peaks,” Ser. No. 09/895,657, filed Jun. 29, 2001. Systemsand techniques are disclosed wherein a gated pilot signal can beacquired faster by checking the neighbor pilot signals of the targetpilot signal.

SUMMARY

[0010] In an aspect of the present invention, a system and method ofre-acquiring a gated pilot signal includes determining the gated pilotsignal is lost and searching a last known pilot offset.

[0011] In an aspect of the present invention, a system and method ofre-acquiring a gated pilot signal includes determining the gated pilotsignal is lost and searching a last coset in which the last pilot signalwas found.

[0012] In another aspect of the present invention, a receiver, comprisesa searcher configured to search for a plurality of pilot signals and aprocessor coupled to the searcher and configured to select a pilotsignal corresponding to the last known pilot offset. In an aspect of thepresent invention, a receiver, comprises a searcher configured to searchfor a plurality of pilot signals and a processor coupled to the searcherand configured to select a pilot signal from a last coset in which thelast pilot signal was found.

[0013] In yet another aspect of the present invention, acomputer-readable medium embodying a program of instructions executableby a computer to perform a method of acquiring a gated pilot signal, themethod comprising determining the gated pilot signal is lost andsearching a last known pilot offset. In an aspect of the presentinvention, a computer-readable medium embodying a program ofinstructions executable by a computer to perform a method of acquiring agated pilot signal, the method comprising determining the gated pilotsignal is lost and searching a last coset in which the last pilot signalwas found.

[0014] It is understood that other embodiments of the present inventionwill become readily apparent to those skilled in the art from thefollowing detailed description, wherein it is shown and described onlyexemplary embodiments of the invention by way of illustration. As willbe realized, the invention is capable of other and different embodimentsand its several details are capable of modification in various otherrespects, all without departing from the spirit and scope of the presentinvention. Accordingly, the drawings and detailed description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Aspects of the present invention are illustrated by way ofexample, and not by way of limitation, in the accompanying drawingswhere:

[0016]FIG. 1 shows an exemplary continuous pilot transmission and agated pilot transmission;

[0017]FIG. 2 is a system diagram of an exemplary communications system;

[0018]FIG. 3 shows an exemplary gated pilot signal;

[0019]FIG. 4 is a timing diagram showing PN code sequences for severalexemplary base stations operating in a CDMA communications system;

[0020]FIG. 5 is a block diagram of an exemplary receiver in a CDMAcommunications system; and

[0021]FIG. 6 is a flow chart illustrating an exemplary algorithmperformed by a processor in a CDMA receiver.

DETAILED DESCRIPTION

[0022] The detailed description set forth below in connection with theappended drawings is intended as a description of exemplary embodimentsof the present invention and is not intended to represent the onlyembodiments in which the present invention can be practiced. In someinstances, the detailed description includes specific details for thepurpose of providing a thorough understanding of the present invention.However, it will be apparent to those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well known structures and devices are shown in blockdiagram form in order to avoid obscuring the concepts of the presentinvention.

[0023] A subscriber station (also called subscriber unit, mobilestation, mobile, remote station, remote terminal, access terminal, anduser equipment), may be mobile or stationary, and may communicate withone or more base stations (BSs) (also called base transceiver systems(BTSs), base station transceivers, access points, access nodes, Node B,and modem pool transceivers (MPTs)).

[0024]FIG. 1 shows an exemplary continuous pilot transmission 20 and agated pilot transmission 22. The gated pilot signal includes a period oftransmitting a pilot signal, followed by a period of not transmitting apilot signal. Gating the pilot signal enables an increase in bandwidthbecause the period of not transmitting a pilot signal can be used totransmit data rather than transmitting the pilot signal.

[0025]FIG. 2 is a system diagram of an exemplary communications system100. Communications system 100 may be designed to support one or moreCDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base StationCompatibility Standard for Dual-Mode Wideband Spread Spectrum CellularSystem” (the IS-95 standard), (2) the “TIA/EIA/IS-856 cdma2000 High RatePacket Data Air Interface Specification” (hereinafter IS-856), (3) thedocuments offered by a consortium named “3rd Generation PartnershipProject” (3GPP) and embodied in a set of documents including DocumentNos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (theW-CDMA standard), and (4) the documents offered by a consortium named“3rd Generation Partnership Project 2” (3GPP2) and embodied in a set ofdocuments including Document Nos. C.S0002-A, C.S0005-A, C.S0010-A,C.S0011-A. C.S0024, C.S0026, C.P9011, and C.P9012 (the cdma2000standard). In the case of the 3GPP and 3GPP2 documents, these areconverted by standards bodies worldwide (e.g., TIA, ETSI, ARIB, TTA, andCWTS) into regional standards and have been converted into internationalstandards by the International Telecommunications Union (ITU). Thesestandards are incorporated herein by reference.

[0026] Although the various aspects of the present invention aredescribed in the context of a CDMA communications system, those skilledin the art will appreciate that the techniques for acquiring a gatedpilot signal described herein are likewise suitable for use in variousother communications environments such as a GSM and TDMA communicationssystems. Accordingly, any reference to a CDMA communications system isintended only to illustrate the inventive aspects of the presentinvention, with the understanding that such inventive aspects have awide range of applications.

[0027] The communications system 100 provides a mechanism for asubscriber station 102 to access a network, or communicate with othersubscriber stations, through one or more base stations. For ease ofexplanation, only three base stations 104, 106 and 108 are shown.However, as a matter of practice, numerous base stations will beoperating with at least one base station located in every cell. Shouldthe cells be divided into sectors, a base station would be located ineach sector. In the described exemplary embodiment, each base station104, 106 and 108 transmits a gated pilot signal 110, 112 and 114,respectively. The gated pilot signal is used by the subscriber station102 for initial synchronization with a base station and to providecoherent demodulation of the transmitted signal once the subscriberstation is synchronized to one of the base stations.

[0028] The gated pilot signal contains no data and is generallycharacterized as an unmodulated spread spectrum signal. The PN code usedto spread each gated pilot signal 110, 112 and 114 should, therefore, bedifferent to allow the subscriber station 102 to distinguish between thethree base stations 104, 106 and 108. The PN code used to spread eachgated pilot signal is known, a priori, by the subscriber station 102,and therefore, each gated pilot signal 110, 112 and 114 can be despreadat the subscriber station through a correlation process with a locallygenerated PN code. A communications channel can then be established withthe base station having the strongest gated pilot signal. Givenrelatively constant environmental conditions, the strongest gated pilotsignal is generally transmitted from the base station in the closestproximity to the receiving subscriber station 102, in this case the basestation 106.

[0029] In an exemplary embodiment of a communications system 100,acquisition of a gated pilot signal can be achieved by employing asearching methodology that exploits certain characteristics of the gatedpilot signal. Pilot search operation consists of correlating theincoming signal with pre-stored Pilot PN sequences and looking forstrong correlation peaks. Once a strong peak is found to be in a coset,the subscriber station zooms in on the PN position of the pilot peak toconfirm the presence of the pilot. After pilot presence is confirmed thesubscriber station tries to achieve frequency lock with the base stationthat is transmitting the peak. After the frequency lock is achieved, thesubscriber station demodulates a control channel to get the timinginformation about the base station transmitting the pilot. Thesubscriber station then adjusts its own timing to synchronize itselfwith the base station.

[0030] Although not limited in applicability, the searching methodologyis particularly adaptable to CDMA communications systems. In CDMAcommunications systems, the gated pilot signal transmitted by each basestation generally has the same PN code but with a different phaseoffset. The use of the same PN code is advantageous because it allows asubscriber station to access a base station with a search through asingle PN code sequence for all phase offsets. The phase offset allowsthe gated pilot signals for each base station to be distinguished fromone another.

[0031] The gated pilot signal transmitted by each base station iscontained in a pilot channel of a forward link waveform. The forwardlink refers to transmissions from a base station to a subscriberstation. The forward link waveform may take on various forms withoutdeparting from the inventive concepts described throughout. By way ofexample, the very nature of a gated pilot signal implies that theforward link channel structure, in its simplest form, includes at leastone channel which is time-division multiplexed with the pilot channel.In the described exemplary embodiment, the pilot channel istime-division multiplexed with a traffic channel. The resulting forwardlink waveform is spread with a PN code, modulated onto a carrierwaveform, amplified and transmitted into its respective cell or sectorby a base station.

[0032] More complex forward link channel structures are alsocontemplated. By way of example, the traffic channel can be parsed intomultiple code channels by spreading each traffic channel with an innerorthogonal code generated by using Walsh functions. Alternatively, thepilot channel can be spread with a Walsh cover, and additional code andtime channels can be added to include a synchronization channel, pagingchannels, and traffic channels.

[0033] In an exemplary embodiment, the communication system 100 has agated pilot signal with a period of 1024 chips. The gated pilot signalincludes a period of transmission for 96 chips followed by a period ofno transmission for 928 chips as shown in FIG. 3.

[0034] In CDMA based communication systems, the base stations areseparated in PN space by integer multiples of a PN increment where onePN increment is 64 PN chips. IS-856 specifies a minimum PN separationbetween two base station PN offsets to be one PN increment, i.e., 64chips.

[0035] In CDMA communication systems, the PN code is periodic andtypically chosen to be 2¹⁵ (32,768) chips per period with 512 PN phaseoffsets spaced apart by 64 chips. Thus, the PN space comprises 2¹⁵(32,768) possible PN positions, which results in a total of 512 (215/64)distinct PN phase offsets possible for a base station. As the gatedpilot period is 1024 chips in an exemplary embodiment, a PN circle has atotal of 32 pilot bursts (215/1024). Thus, the pilot signal is spread bythe PN code and transmitted in 32 pilot signal bursts per period.

[0036] A continuous pilot signal is a repeating series of symbols asshown by “PN 0,” “PN 1,” “PN 2,” “PN 3,” and “PN 4” in FIG. 4. FIG. 4 isa timing diagram for an exemplary communications system showing five PNcodes 202 each being 32,768 chips long. For ease of explanation, theterm “symbol” will be used as a shorthand convention for identifying a64-chip PN code sequence with the understanding that the gated pilotsignal contains no data. Using this convention, the 32,768-chip PN codecan be represented by a 512 symbol sequence.

[0037] In the following example, a system having only 64 distinct PNphase offsets is assumed. Each PN code includes the same symbol sequencebut offset in phase. By way of example, PN0 is offset by one symbol fromPN1. Similarly, PN1 is offset by one symbol from PN2, PN2 is offset byone symbol from PN3, and PN3 is offset by one symbol from PN4.Neighboring base stations would transmit the same pilot signal, butstarting at a different offset in the sequence, as shown by “PN 1”through “PN 4”. Since there are 64 symbols in this example, there wouldbe a maximum of 64 distinct PN offsets, 0 through 63.

[0038] Each PN code is used to spread a pilot signal. A gating function204 is then applied to each spread spectrum pilot signal 202. Forpurposes of illustration, the gating function will be defined as a gatehaving a one-symbol width and a four-symbol period. As a result of thisgating function 204, four different symbol sequences 206 are generatedPN 0, PN1, PN 2, and PN 3. The same symbol sequence is generated everyfourth PN code phase offset as shown by PN0 and PN4. All gated pilotsignals having the same symbol sequence, regardless of phase shift, canbe grouped together into sets known as a coset as follows:

[0039] Coset0: PN 0, PN4, PN 8, . . . PN 508

[0040] Coset1: PN1, PN 3, PN 9, . . . , PN 509

[0041] Coset2: PN 2, PN6, PN 10, . . . PN 510

[0042] Coset3: PN 3, PN7, PN11, . . . PN 511

[0043] where the number of cosets can be defined as the number of PNcode phase offsets divided by the number of pilot signal bursts perperiod. Coset is defined as the set of pilot offsets which share thesame set of symbols, shifted in time. When searching for the pilotsignal bursts from one coset, the pilot signal bursts from the othercosets will not be seen.

[0044] In other communications systems, the length of the PN code forspreading the pilot signal may vary depending on a variety of factors. Ashort PN code facilitates faster acquisition time whereas a long PN codeincreases code processing gain. Those skilled in the art will be readilyable to assess the performance tradeoffs to determine the optimal lengthfor the PN code. In addition, the number of phase offsets, spacings, andpilot bursts per period can be varied to optimize system parameters.

[0045] In an exemplary embodiment, pilots, which are separated bymultiples of the gated pilot period, such as every 16 (1024/64) PNoffsets, will appear to have identical pilot bursts, although shifted intime. Table 1 shows all the possible 512 PN offsets divided into 16different cosets. TABLE 1 PN offsets divided into Cosets Coset # PNOffsets Coset0 0, 16, 32, 48, 64, ....496 Coset1 1, 17, 33, 49, 65,....497 Coset2 2, 18, 34, 50, 66, ....498 Coset3 3, 19, 35, 51, 67,....499 . . . . . . Coset15 15, 31, 47, 63, 79, ....523

[0046] To have a larger separation in the PN space for adjacent basestations, it is likely that the network operators would prefer usinghigher PN increments to lower PN increments. Some CDMA network operatorsthat prefer using a PN increment of four for most networks mayoccasionally switch to a PN increment of two or one for denselypopulated networks. Thus, searching cosets in decreasing order of PNincrement may result in a reduction in acquisition time.

[0047] In an exemplary embodiment, four cosets are gathered into onegroup, thus dividing the 16 cosets as shown in Table 1 into fourdifferent coset groups as shown in Table 2, where for CN, N is avariable that denotes the coset number. TABLE 2 Coset Groups Coset Group0 C0, C4, C8, C12 Coset Group 1 C2, C6, C10, C14 Coset Group 2 C3, C7,C11, C15 Coset Group 3 C1, C5, C9, C13

[0048] Coset Group 0 (CG0) contains offsets for PN increments which arean integer multiple of four. CG1 contains offsets for PN incrementswhich are an integer multiples of two excluding entries from CG0. CG2and CG3 contain the remaining odd PN offsets. To enhance the probabilityof early detection of the pilot signal, a subscriber station can searchthe cosets in the order CG0, CG1, CG2 and CG3.

[0049] There are many occasions where a subscriber station looses apilot signal and has to re-acquire the pilot by going through pilotre-acquisition. A system loss exception occurs when a subscriber stationloses the pilot signal. IS-856 pilot search operation includes searchingan entire PN sequence for each of the 16 cosets sequentially, whichmakes the IS-856 pilot searching operation at least 16 times more timeconsuming than CDMA pilot search operation of a continuous CDMA pilotsignal.

[0050] Once pilot search has found the pilot signal, then the subscriberstation goes through a series of timing synchronization procedures toalign the subscriber station's timing and frequency with the basestation. Once the timing synchronization is achieved, the subscriberstation declares a successful acquisition and enters steady stateoperation. In the steady state operation, the subscriber stationcontinually tracks the pilot by searching it periodically. Pilotacquisition searches take an order of magnitude longer than the periodictracking searches performed in the steady state.

[0051] During steady state operation of the subscriber station, thesubscriber station continually tracks the PN phase of the pilot signalin order to coherently demodulate the associated data channels andmaintain the system timing. There are many occasions that could resultin the subscriber station losing the system timing and declaring systemloss. A common reason for system loss is not being able to track a pilotfor an extended period of time.

[0052] Another reason for system loss is due to pilot fade. If thesubscriber station is moving away from coverage then the pilot signalslowly fades into noise. In an IS-856 embodiment, if the pilot signal isnot found for a certain period of time, then the various supervisiontimers defined in IS-856 protocol expire, which causes the subscriberstation to generate system loss. Pilot fade is also possible due totemporary loss of pilot signal when the subscriber station enters anelevator or tunnel.

[0053] One of the less common occurrences of system loss is diagnosis ofa fault in the subscriber station. When encountered with a problem thatprohibits normal operation of the subscriber station, the subscriberstation diagnostic software typically pauses the software to work aroundor fix the fault. This results in losing the timing synchronization withthe base station resulting in system loss.

[0054] System losses are very common in subscriber stations that performCDMA+IS-856 hybrid operation sharing a single RF chain intime-multiplexed fashion. The reason is that these hybrid devices givehigher priority to delay sensitive CDMA operation than delay tolerantIS-856 operation. Hence any prolonged CDMA activity results in IS-856operation not being able to track the pilot signal, which results insystem loss followed by system re-acquisition. Examples of a few suchCDMA activities are given below.

[0055] Mobile Registration on a CDMA network

[0056] Voice page reception or voice call origination from a CDMAnetwork

[0057] Short messaging service activity with CDMA network

[0058] Position location session using CDMA pilot signals

[0059] The above mentioned hybrid mode activities can occur oftendepending upon the way the hybrid subscriber station is being used bythe user. Every time such system loss occurs IS-856 operation couldspend a long time re-acquiring the base station.

[0060] In an embodiment, the subscriber station enters a pilotacquisition state and begins the process of acquiring the pilot signalby searching cosets after declaring a system loss exception. This iscalled a re-acquisition of the pilot signal since the subscriber stationhad already acquired and lost the pilot signal. As the AT had alreadyacquired a pilot before and had synchronized its timing to a basestation, the time it takes to re-acquire the pilot signal can be reducedusing the knowledge of the last known PN offset.

[0061]FIG. 5 is a block diagram of an exemplary receiver in a subscriberstation operating in a CDMA communications system. In the describedexemplary embodiment, the signal transmissions from all the basestations are received through one or more antennas 302. The resultingsuperimposed signal received by the antenna 302 is provided to an RFsection 304. The RF section 304 filters and amplifies the signal,downconverts the signal to baseband, and digitizes the baseband signal.The digital samples are provided to memory 306 for the purposes ofacquisition. The memory 306 stores the number of chips equal to orgreater than the period of the pilot signal burst. This approach shouldresult in at least one gated pilot burst from each base station beingcaptured in memory 306. An HDR communications system with 32 pilotsignal bursts over a PN code sequence of 32,768 chips has a pilot signalburst period equal to 1024 chips.

[0062] The acquisition process involves searching through the digitalsamples stored in memory to find all the pilot signal bursts for onecoset. This can be achieved by correlating the digital samples stored inmemory with a locally generated PN code sequence. By way of example, asearcher 308 generates a symbol, i.e., a 64 chip PN code sequence,common to the gated pilot signals from each base station in the samecoset. The symbol from the searcher 308 is coupled to a demodulator 310where it is correlated with the digital samples stored in memory 306.The searcher 308 sequentially shifts the symbol in phase as part of asystematic search through the digital samples to find a correspondingsymbol in memory 306.

[0063] The demodulator 310 can be implemented in a variety of fashions.By way of example, in CDMA communications systems, or any other type ofcommunications system, which uses diversity techniques to combat fading,a RAKE receiver may be used. The RAKE receiver in a CDMA communicationssystem typically utilizes independent fading of resolvable multipaths toachieve diversity gain. Specifically, the RAKE receiver can beconfigured to process one or more multipaths of the gated pilot signal.Each multipath signal is fed into a separate finger processor to performPN code despreading with the locally generated PN code from the searcher308. Walsh code decovering may also be provided by the RAKE receiver ifneeded. The RAKE receiver then combines the output from each fingerprocessor to recover the gated pilot signal.

[0064] The output of the demodulator 310 is provided to a processor 312.The processor 312 is coupled to the searcher 308 and implements anacquisition algorithm to select the base station having the strongestpilot signal.

[0065] An acquisition algorithm implemented by the processor inaccordance with an embodiment is illustrated by the flow chart of FIG.6. In step 602, the processor enters a pilot acquisition state. In step604, a variable coset representing a coset group is initialized to zero.In step 606, the processor searches coset group n, where the value n isan integer.

[0066] In step 608, a check is made to determine whether the pilot wasfound in the current coset n. If the pilot was found, then in step 616,the subscriber station is synchronized with the base station of thefound pilot. Otherwise, the flow of control proceeds to step 610.

[0067] In step 610, a check is made to determine whether all of thecosets have been searched. In the embodiment of FIG. 6, the number ofcosets searched is 16. It would be apparent to those skilled in the artthat other embodiments may have a different number of cosets to search.If all of the cosets have been searched, then the flow of controlproceeds to step 614. In step 614, the pilot search has failed isindicated. If not all of the cosets have been searched, then the flow ofcontrol proceeds to step 612. In step 612, the coset n is incremented,which means that the next coset shall be searched and the flow ofcontrol proceeds to step 606.

[0068] After the subscriber station is synchronized with the basestation of the found pilot in step 616, the flow of control proceeds tostep 618 and the subscriber station is in steady state operation.

[0069] When there is a system loss exception, the flow of controlproceeds to step 620. In step 620, a check is made to determine whetherthe system loss was due to fading. If the system loss is due to fading,then the flow of control proceeds to step 622, otherwise the flow ofcontrol proceeds to step 626.

[0070] In step 622, the subscriber station searches the last PN offset.The subscriber station searches the last known pilot offset using asmall search window on the PN sequence. If the pilot was foundsuccessfully, then the subscriber station's timing is intact and thesubscriber station does not need to go through a complete coset search.Thus, re-acquisition is achieved in a smaller time span than requiredfor a complete system acquisition and the flow of control proceeds tostep 618.

[0071] If no pilot was found during the small window search around thelast known PN offset, then it could be due to the pilot slipping too fardue to lack of tracking. The flow of control proceeds to step 626. Instep 626 a quick search of the last coset is performed. The coset inwhich the last pilot was found is searched. The subscriber stationperforms an entire PN sequence search within the last coset looking forthe last known PN offset. The flow of control proceeds to step 628.

[0072] In step 628, a check is made to determine whether the pilot wasfound. If the pilot was found, then the flow of control proceeds to step616 and the subscriber station is synchronized with the base station ofthe found pilot. If the pilot was not found, then the flow of controlproceeds to step 602.

[0073] If the pilot was not found in both the above quick searches, thenthe subscriber station can enter the pilot acquisition state to performpilot search of all cosets. The overhead for the two quick searches isonly a fraction of the total time needed for complete systemacquisition. Thus, re-acquisition utilizing the two quick searchesspeeds up the acquisition process.

[0074] Although the exemplary processor algorithm described inconnection with FIG. 6 includes various sequential steps, those skilledin the art will appreciate that the sequence of steps may be altered tooptimize processor resources, or alternatively, one or more steps may beprocessed in parallel. Moreover, one or more steps could be omitted, oradditional steps known in the art could be used, either alone or incombination with one or more of the described algorithmic steps setforth in FIG. 6.

[0075] It would be apparent to those skilled in the art that embodimentsmay include one or both of the re-acquisition search techniques shown inFIG. 6. In an embodiment, a gated pilot signal can be re-acquired fasterby searching a last known pilot offset. In an embodiment, a gated pilotsignal can be re-acquired faster by searching a last coset in which thelast pilot signal was found.

[0076] Those skilled in the art will appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the embodiments disclosed herein may be implementedas electronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, andalgorithms have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

[0077] The various illustrative logical blocks, modules, and circuitsdescribed in connection with the embodiments disclosed herein may beimplemented or performed with a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

[0078] The methods or algorithms described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

[0079] The previous description of the disclosed embodiments is providedto enable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of re-acquiring a gated pilot signal,comprising: determining the gated pilot signal is lost; and searching alast known pilot PN offset.
 2. The method of claim 1, further comprisingentering steady state operation if the gated pilot signal is found atthe last known pilot PN offset.
 3. The method of claim 1, furthercomprising searching a last coset in which the last pilot signal wasfound.
 4. The method of claim 3, wherein searching a last coset in whichthe last pilot signal was found involves performing an entire PNsequence search within the last coset.
 5. The method of claim 4, furthercomprising entering steady state operation if the gated pilot signal isfound in the last coset in which the last pilot signal was found.
 6. Themethod of claim 4, further comprising searching other cosets if thegated pilot signal is not found in the last coset in which the lastpilot signal was found.
 7. A method of re-acquiring a gated pilotsignal, comprising: determining the gated pilot signal is lost; andsearching a last coset in which the last pilot signal was found.
 8. Amethod of re-acquiring a gated pilot signal, comprising: determining thegated pilot signal is lost; and searching a last coset in which the lastpilot signal was found.
 9. A receiver, comprising: a searcher configuredto search for a plurality of pilot signals; and a processor coupled tothe searcher and configured to select a pilot signal corresponding tothe last known pilot PN offset.
 10. A receiver, comprising: a searcherconfigured to search for a plurality of pilot signals; and a processorcoupled to the searcher and configured to select a pilot signal from alast coset in which the last pilot signal was found.
 11. Acomputer-readable medium embodying a program of instructions executableby a computer to perform a method of acquiring a gated pilot signal, themethod comprising: determining the gated pilot signal is lost; andsearching a last known pilot PN offset.
 12. A computer-readable mediumembodying a program of instructions executable by a computer to performa method of acquiring a gated pilot signal, the method comprising:determining the gated pilot signal is lost; and searching a last cosetin which the last pilot signal was found.